Blastocyst implantation involves complex synchronized interactions between the heterogeneous cell types of the uterus and embryo. Failure of embryo implantation is a major clinical problem that accounts for the intense research to identify the signals or mediators responsible for successful contact of the blastocyst to the uterine wall of the mother. Histamine, a well-known regulator of inflammation, immune reactions, vasodilatation, cell growth and differentiation, has long been suspected to play an important role in establishing embryo-uterine communication during implantation. Uterine mast cells have been considered as the major source of histamine. The P.I.'s recent investigation in the mouse demonstrated for the first time that the uterine epithelial cells, but not the resident mast cells, are the major source of histamine. Major expression of histidine decarboxylase (HDC) that produces histamine occurs in the uterine epithelium on day 4 of pregnancy (the day of implantation), and the uterine HDC gene is regulated by progesterone in the mouse. Histamine actions are mediated by histamine receptors (H1, H2, and H3). The P.I.'s preliminary results suggest that day 4 blastocysts express the H2 receptor gene, but not the HDC or H1 receptor genes. These findings of expression of H2 receptors in blastocysts, and histamine synthesis in luminal epithelial cells suggest that blastocysts are a target for histamine ligand-receptor signaling. Therefore, the following specific aims are proposed. The investigators will study in mice: (1) Mechanisms of progesterone regulation of HDC expression in the uterus; (2) Expression of H2 receptors in normal, dormant and activated blastocyst; and (3) Role of histamine ligand-receptor interactions in blastocyst development and implantation. Multiple experimental approaches are proposed including reverse transcriptase-polymerase chain reaction (RT-PCR), Northern hybridization, nuclear run-off transcription assay, in situ hybridization, immunostaining, Western blotting, ligand binding studies, autoradiographic localization of ligand-binding sites, enzyme assay, embryo culture and transfer, and others to accomplish the goals. The results will further knowledge of cell-cell interactions via histamine ligand-receptor signaling in implantation.